DEPOSITIONAL AND PALEOGEOGRAPHIC SETTING OF PHOSPHORITES AND METALLIFEROUS BLACK SHALES IN THE CARBONIFEROUS-PERMIAN LISBURNE GROUP, NORTHERN ALASKA

Phosphorites are widely distributed in the Lisburne Group, a mainly Carboniferous carbonate succession that occurs throughout northern Alaska. New sedimentologic, geochemical, and isotopic data constrain the depositional and paleogeographic setting of these rocks. Lisburne phosphorites are granular to nodular, Late Mississippian (mainly early Chesterian), and formed primarily in suboxic outer ramp settings. Where best developed, they cap regressive parasequences of lime mudstone and black shale and occur through a zone ≤12 m thick. High gamma-ray response through this zone indicates strongly condensed facies due to sediment starvation and development of phosphatic hardgrounds; phosphorite textures record multiple episodes of phosphogenesis.

Geochemical and isotopic data from black shales and intercalated phosphorites suggest deposition under suboxic, denitrifying conditions. Black shales contain up to 20 wt % C_org and have high values of Cr, V, Ni, Zn, Cu, Ag, and Tl. Very high V/Mo, Cr/Mo, and Re/Mo ratios (all marine fractions, MF) and low MnO contents (<0.01 wt %) characterize the black shales. Cr_MF/V_MF ratios (mostly 0.8-1.9) and high δ¹⁵N values indicate moderate to strong denitrification. Most black shales and all granular phosphorites have large negative Ce anomalies that likely reflect deposition in seawater depleted in Ce due to increased oxygenation of the biosphere during Carboniferous evolution of land plants. A negative excursion in carbonate δ¹³C characterizes the phosphorite interval; similar excursions are seen in other phosphorites, including the nearly coeval (Lower Mississippian) Delle Phosphatic Member in the western interior United States.

Paleogeographic reconstructions of northern Alaska imply that Lisburne phosphorites formed in the Ikpikpuk basin and along both sides of the Kuna basin, which hosts giant massive sulfide and barite deposits of the Red Dog district. Lisburne phosphatic strata are coeval with these deposits and formed as part of a high-productivity upwelling regime. Recent global reconstructions are consistent with Carboniferous upwelling in northern Alaska but differ in the type of upwelling expected (zonal versus meridional). Paleoenvironmental data suggest that meridional upwelling better explains phosphorite deposition in the Lisburne Group.